In control devices of internal combustion engines using neural networks, there is known a control device of an internal combustion engine designed to learn in advance the weight of a neural network based on values of an engine speed, amount of intake air, and other operating parameters of the engine so that an amount of gas sucked into a combustion chamber matches an actual amount of gas sucked into the combustion chamber and to use the neural network with the learned weight at the time of engine operation so as to estimate the amount of gas sucked into the combustion chamber from the values of the operating parameters of the engine (for example, see Japanese Patent Publication No. 2012-112277A).